检查了我的 JSON 获取是否正确，没有错误，解析完美完成，但应用程序没有显示任何结果(显示空 ListView )。 如果有人可以帮助我解决这个问题，请提供我的所有文件，那就太好了。 注意:该应用程序没有显示任何错误，它只是显示空白 ListView 。
WordActivity.java
package com.example.anandparmeetsingh.books;
import android.app.LoaderManager;
import android.content.Context;
import android.content.Loader;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ListView;
import android.widget.TextView;
import java.util.ArrayList;
import java.util.List;
public class WordActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<List<Word>> {
private static final int EARTHQUAKE_LOADER_ID = 1;
private static final String LOG_TAG = WordActivity.class.getName();
/**
* URL for earthquake data from the USGS dataset
*/
private static final String USGS_REQUEST_URL =
"https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&starttime=2014-01-01&endtime=2014-01-02";
/**
* Adapter for the list of earthquakes
*/
public WordAdapter mAdapter;
private TextView mEmptyStateTextView;
@Override
protected void onCreate(Bundle bundle) {
super.onCreate(bundle);
setContentView(R.layout.activity_word);
// Get a reference to the LoaderManager, in order to interact with loaders.
mAdapter = new WordAdapter(this, new ArrayList<Word>());
ListView earthquakeListView = (ListView) findViewById(R.id.list);
// Create a new adapter that takes an empty list of earthquakes as input
final WordAdapter adapter = new WordAdapter(this, new ArrayList<Word>());
// Set the adapter on the {@link ListView}
// so the list can be populated in the user interface
earthquakeListView.setAdapter(mAdapter);
// Set an item click listener on the ListView, which sends an intent to a web browser
// to open a website with more information about the selected earthquake.
// Start the AsyncTask to fetch the earthquake data
//mEmptyStateTextView = (TextView) findViewById(R.id.empty_view);
//earthquakeListView.setEmptyView(mEmptyStateTextView);
ConnectivityManager connMgr = (ConnectivityManager)
getSystemService(Context.CONNECTIVITY_SERVICE);
// Get details on the currently active default data network
NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
// If there is a network connection, fetch data
if (networkInfo != null && networkInfo.isConnected()) {
// Get a reference to the LoaderManager, in order to interact with loaders.
LoaderManager loaderManager = getLoaderManager();
// Initialize the loader. Pass in the int ID constant defined above and pass in null for
// the bundle. Pass in this activity for the LoaderCallbacks parameter (which is valid
// because this activity implements the LoaderCallbacks interface).
loaderManager.initLoader(EARTHQUAKE_LOADER_ID, null, this);
} else {
// Otherwise, display error
// First, hide loading indicator so error message will be visible
//View loadingIndicator = findViewById(R.id.loading_indicator);
//loadingIndicator.setVisibility(View.GONE);
//Update empty state with no connection error message
//mEmptyStateTextView.setText("No Connection");
}
}
public Loader<List<Word>> onCreateLoader(int i, Bundle bundle) {
return new WordLoader(this, USGS_REQUEST_URL);
}
/**
* {@link } to perform the network request on a background thread, and then
* update the UI with the list of earthquakes in the response.
* <p>
* AsyncTask has three generic parameters: the input type, a type used for progress updates, and
* an output type. Our task will take a String URL, and return an EarthquakeAdapter. We won't do
* progress updates, so the second generic is just Void.
* <p>
* We'll only override two of the methods of AsyncTask: doInBackground() and onPostExecute().
* The doInBackground() method runs on a background thread, so it can run long-running code
* (like network activity), without interfering with the responsiveness of the app.
* Then onPostExecute() is passed the result of doInBackground() method, but runs on the
* UI thread, so it can use the produced data to update the UI.
*/
@Override
public void onLoadFinished(Loader<List<Word>> loader, List<Word> earthquakes) {
// Clear the adapter of previous earthquake data
//View loadingIndicator = findViewById(R.id.loading_indicator);
//loadingIndicator.setVisibility(View.GONE);
//mEmptyStateTextView.setText(R.string.no_earthquakes);
mAdapter.clear();
// If there is a valid list of {@link Earthquake}s, then add them to the adapter's
// data set. This will trigger the ListView to update.
if (earthquakes != null && !earthquakes.isEmpty()) {
mAdapter.addAll(earthquakes);
}
}
@Override
public void onLoaderReset(Loader<List<Word>> loader) {
// Loader reset, so we can clear out our existing data.
mAdapter.clear();
}
}
Word.java
package com.example.anandparmeetsingh.books;
public class Word {
private String mMagnitude;
private String mLocation;
private Long mDate;
private String mUrl;
public Word(String magnitude, String location, String url) {
mMagnitude = magnitude;
mLocation = location;
mUrl = url;
}
public Word(String magnitude, String location, Long date, String url) {
mMagnitude = magnitude;
mLocation = location;
mDate = date;
mUrl = url;
}
public String getMagnitude() {
return mMagnitude;
}
public String geLocation() {
return mLocation;
}
public Long getDate() {
return mDate;
}
public String getUrl(){return mUrl;}
}
WordAdapter.java
package com.example.anandparmeetsingh.books;
import android.app.Activity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;
import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
/**
* Created by ParmeetSingh on 5/9/2017.
*/
public class WordAdapter extends ArrayAdapter<Word> {
String primaryLocation;
String locationOffset;
String description;
private static final String LOCATION_SEPARATOR = " of ";
public WordAdapter(Activity context, ArrayList<Word> words) {
// Here, we initialize the ArrayAdapter's internal storage for the context and the list.
// the second argument is used when the ArrayAdapter is populating a single TextView.
// Because this is a custom adapter for two TextViews and an ImageView, the adapter is not
// going to use this second argument, so it can be any value. Here, we used 0.
super(context, 0, words);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
// Check if the existing view is being reused, otherwise inflate the view
View listItemView = convertView;
if (listItemView == null) {
listItemView = LayoutInflater.from(getContext()).inflate(
R.layout.activity_list_display, parent, false);
}
// Get the {@link AndroidFlavor} object located at this position in the list
Word currentWord = getItem(position);
// Find the TextView with view ID magnitude
TextView magnitudeView = (TextView) listItemView.findViewById(R.id.magnitude);
// Format the magnitude to show 1 decimal place
// Display the magnitude of the current earthquake in that TextView
magnitudeView.setText(description);
String originalLocation = currentWord.geLocation();
// Find the TextView in the list_item.xml layout with the ID version_number
TextView primaryLocationView = (TextView) listItemView.findViewById(R.id.primary_location);
// Display the location of the current earthquake in that TextView
primaryLocationView.setText(primaryLocation);
// Find the TextView with view ID location offset
TextView locationOffsetView = (TextView) listItemView.findViewById(R.id.location_offset);
// Display the location offset of the current earthquake in that TextView
locationOffsetView.setText(locationOffset);
//Date dateObject = new Date(currentWord.getDate());
// Find the TextView with view ID date
TextView dateView = (TextView) listItemView.findViewById(R.id.date);
// Format the date string (i.e. "Mar 3, 1984")
//String formattedDate = formatDate(dateObject);
// Display the date of the current earthquake in that TextView
dateView.setText(primaryLocation);
// Find the TextView with view ID time
TextView timeView = (TextView) listItemView.findViewById(R.id.time);
// Format the time string (i.e. "4:30PM")
//String formattedTime = formatTime(locationOffset);
// Display the time of the current earthquake in that TextView
timeView.setText(locationOffset);
return listItemView;
}
private String formatMagnitude(double magnitude) {
DecimalFormat magnitudeFormat = new DecimalFormat("0.0");
return magnitudeFormat.format(magnitude);
}
/**
* Return the formatted date string (i.e. "Mar 3, 1984") from a Date object.
*/
private String formatDate(Date dateObject) {
SimpleDateFormat dateFormat = new SimpleDateFormat("LLL dd, yyyy");
return dateFormat.format(dateObject);
}
private String formatTime(Date dateObject) {
SimpleDateFormat timeFormat = new SimpleDateFormat("h:mm a");
return timeFormat.format(dateObject);
}
}
WordLoader.java
package com.example.anandparmeetsingh.books;
import android.content.AsyncTaskLoader;
import android.content.Context;
import java.util.List;
/**
* Created by ParmeetSingh on 5/9/2017.
*/
public class WordLoader extends AsyncTaskLoader<List<Word>> {
/** Tag for log messages */
private static final String LOG_TAG = WordLoader.class.getName();
/** Query URL */
private String mUrl;
/**
* Constructs a new {@link WordLoader}.
*
* @param context of the activity
* @param url to load data from
*/
public WordLoader(Context context, String url) {
super(context);
mUrl = url;
}
@Override
protected void onStartLoading() {
forceLoad();
}
/**
* This is on a background thread.
*/
@Override
public List<Word> loadInBackground() {
if (mUrl == null) {
return null;
}
// Perform the network request, parse the response, and extract a list of earthquakes.
List<Word> earthquakes = QueryUtils.fetchEarthquakeData(mUrl);
return earthquakes;
}
}
QueryUtils.java
package com.example.anandparmeetsingh.books;
import android.text.TextUtils;
import android.util.Log;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;
/**
* Created by ParmeetSingh on 5/9/2017.
*/
public class QueryUtils {
/**
* Tag for the log messages
*/
private static final String LOG_TAG = QueryUtils.class.getSimpleName();
/**
* Create a private constructor because no one should ever create a {@link QueryUtils} object.
* This class is only meant to hold static variables and methods, which can be accessed
* directly from the class name QueryUtils (and an object instance of QueryUtils is not needed).
*/
private QueryUtils() {
}
/**
* Query the USGS dataset and return a list of {@link WordAdapter} objects.
*/
public static List<Word> fetchEarthquakeData(String requestUrl) {
// Create URL object
URL url = createUrl(requestUrl);
// Perform HTTP request to the URL and receive a JSON response back
String jsonResponse = null;
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
jsonResponse = makeHttpRequest(url);
} catch (IOException e) {
Log.e(LOG_TAG, "Problem making the HTTP request.", e);
}
// Extract relevant fields from the JSON response and create a list of {@link EarthquakeAdapter}s
List<Word> earthquake = extractFeatureFromJson(jsonResponse);
// Return the list of {@link EarthquakeAdapter}s
return earthquake;
}
/**
* Returns new URL object from the given string URL.
*/
private static URL createUrl(String stringUrl) {
URL url = null;
try {
url = new URL(stringUrl);
} catch (MalformedURLException e) {
Log.e(LOG_TAG, "Problem building the URL ", e);
}
return url;
}
/**
* Make an HTTP request to the given URL and return a String as the response.
*/
private static String makeHttpRequest(URL url) throws IOException {
String jsonResponse = "";
// If the URL is null, then return early.
if (url == null) {
return jsonResponse;
}
HttpURLConnection urlConnection = null;
InputStream inputStream = null;
try {
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setReadTimeout(10000 /* milliseconds */);
urlConnection.setConnectTimeout(15000 /* milliseconds */);
urlConnection.setRequestMethod("GET");
urlConnection.connect();
// If the request was successful (response code 200),
// then read the input stream and parse the response.
if (urlConnection.getResponseCode() == HttpURLConnection.HTTP_OK) {
inputStream = urlConnection.getInputStream();
jsonResponse = readFromStream(inputStream);
} else {
Log.e(LOG_TAG, "Error response code: " + urlConnection.getResponseCode());
}
} catch (IOException e) {
Log.e(LOG_TAG, "Problem retrieving the earthquake JSON results.", e);
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
if (inputStream != null) {
// Closing the input stream could throw an IOException, which is why
// the makeHttpRequest(URL url) method signature specifies than an IOException
// could be thrown.
inputStream.close();
}
}
return jsonResponse;
}
/**
* Convert the {@link InputStream} into a String which contains the
* whole JSON response from the server.
*/
private static String readFromStream(InputStream inputStream) throws IOException {
StringBuilder output = new StringBuilder();
if (inputStream != null) {
InputStreamReader inputStreamReader = new InputStreamReader(inputStream, Charset.forName("UTF-8"));
BufferedReader reader = new BufferedReader(inputStreamReader);
String line = reader.readLine();
while (line != null) {
output.append(line);
line = reader.readLine();
}
}
return output.toString();
}
/**
* Return a list of {@link WordAdapter} objects that has been built up from
* parsing the given JSON response.
*/
private static List<Word> extractFeatureFromJson(String earthquakeJSON) {
// If the JSON string is empty or null, then return early.
if (TextUtils.isEmpty(earthquakeJSON)) {
return null;
}
// Create an empty ArrayList that we can start adding earthquakes to
List<Word> earthquakes = new ArrayList<>();
// Try to parse the JSON response string. If there's a problem with the way the JSON
// is formatted, a JSONException exception object will be thrown.
// Catch the exception so the app doesn't crash, and print the error message to the logs.
try {
// Create a JSONObject from the JSON response string
JSONObject baseJsonResponse = new JSONObject(earthquakeJSON);
// Extract the JSONArray associated with the key called "features",
// which represents a list of features (or earthquakes).
JSONArray earthquakeArray = baseJsonResponse.getJSONArray("features");
// For each earthquake in the earthquakeArray, create an {@link EarthquakeAdapter} object
for (int i = 0; i < earthquakeArray.length(); i++) {
// Get a single earthquake at position i within the list of earthquakes
JSONObject currentEarthquake = earthquakeArray.getJSONObject(i);
// For a given earthquake, extract the JSONObject associated with the
// key called "properties", which represents a list of all properties
// for that earthquake.
JSONObject properties = currentEarthquake.getJSONObject("properties");
// Extract the value for the key called "mag"
String magnitude = properties.getString("mag");
// Extract the value for the key called "place"
String location = properties.getString("place");
// Extract the value for the key called "time"
long time = properties.getLong("time");
// Extract the value for the key called "url"
String url = properties.getString("url");
// Create a new {@link EarthquakeAdapter} object with the magnitude, location, time,
// and url from the JSON response.
Word earthquake = new Word(magnitude, location, time, url);
// Add the new {@link EarthquakeAdapter} to the list of earthquakes.
earthquakes.add(earthquake);
}
} catch (JSONException e) {
// If an error is thrown when executing any of the above statements in the "try" block,
// catch the exception here, so the app doesn't crash. Print a log message
// with the message from the exception.
Log.e("QueryUtils", "Problem parsing the earthquake JSON results", e);
}
// Return the list of earthquakes
return earthquakes;
}
}
activity_list_display.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:paddingEnd="16dp"
android:paddingLeft="16dp"
android:paddingRight="16dp"
android:paddingStart="16dp"
tools:context="com.example.a.books.Word">
<TextView
android:id="@+id/magnitude"
android:layout_width="36dp"
android:layout_height="36dp"
android:textColor="#000000"
android:layout_gravity="center_vertical"
android:fontFamily="sans-serif-medium"
android:gravity="center"
android:layout_margin="5dp"
android:textSize="16sp"
tools:text="8.9" />
<TextView
android:id="@+id/location_offset"
android:textColor="#000000"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:ellipsize="end"
android:fontFamily="sans-serif-medium"
android:maxLines="1"
android:textAllCaps="true"
android:textSize="12sp"
tools:text="30km S of" />
<TextView
android:id="@+id/primary_location"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:ellipsize="end"
android:textColor="#000000"
android:maxLines="2"
android:textSize="16sp"
tools:text="Long placeholder location that should wrap to more than 2 lines of text" />
<TextView
android:id="@+id/date"
android:textColor="#000000"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="end"
android:textSize="12sp"
tools:text="Mar 6, 2010" />
<TextView
android:id="@+id/time"
android:textColor="#000000"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="end"
android:textSize="12sp"
tools:text="3:00 PM" />
</LinearLayout>
activity_word.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal">
<ListView
android:id="@+id/list"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" />
</RelativeLayout>
AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.a.books">
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET"/>
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".WordActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".Word"></activity>
</application>
</manifest>
最佳答案
使用
@Override
public void onLoadFinished(Loader<List<Word>> loader, List<Word> earthquakes) {
mAdapter.clear();
if (earthquakes != null && !earthquakes.isEmpty()) {
mAdapter = new WordAdapter(this, earthquakes)
}
if (earthquakeListView.getAdapter() != null) {
if (earthquakeListView.getAdapter().getCount() == 0) {
earthquakeListView.setAdapter(mAdapter);
} else {
mAdapter.notifyDataSetChanged();
}
} else
{
earthquakeListView.setAdapter(mAdapter);
}
}
关于java - JSON 加载/获取正确但无法显示，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/43884771/